1. Field of the Invention
The present invention relates to a flow control valve. In particular, the present invention is directed to a solenoid flow control valve having a resiliently-supported plunger assembly for frictionless operation, which is capable of consistent sealing, low leakage and long cycle life.
2. Description of the Prior Art
A variety of valve configurations are available for controlling fluid flow. One such configuration utilizes a sliding-fit plunger within the chamber of a valve body. Typically, the valve body includes an inlet and outlet port, and a sealing interface against which the sliding-fit plunger is adapted to engage during operation of the flow control valve to prevent fluid flow. This sliding-fit plunger assembly is commonly utilized within conventional solenoid flow control valves.
A conventional solenoid flow control valve includes an electromagnet having a solenoid coil wrapped around an open core spool. The core spool includes a bore hole extending therethrough, the bore hole being of sufficient diameter to permit substantially uninhibited sliding movement of the plunger therein. Of course, to permit such uninhibited movement, an annular gap must be provided between the plunger and the bore hole such that the sliding-fit plunger is capable of axial, radial and rotational movement. With the plunger provided so as to slide within the bore hole of the core spool, axial movement of the plunger may then be controlled by energizing the wire coil to induce a magnetic flux through the open core spool. In this manner, the sliding-fit plunger is drawn into the bore hole, and a magnetic flux path or magnetic circuit is generally established through the sliding-fit plunger, the core spool, and the solenoid housing which substantially encloses the wire coil.
Several embodiments of a solenoid flow control valve having a sliding-fit plunger assembly include U.S. Pat. Nos. 4,621,788 issued to Delew et al.; 4,641,072 issued to Cummins; 4,655,249 issued to Livet; 4,682,675 issued Eddy, Jr.; 4,691,621 issued to Hall; 4,746,094 issued to Cummins; 4,753,212 issued to Miyaki et al.; 4,765,587 issued to Cummins; 4,782,862 issued to Nguyen; and 5,015,000 issued to Perini.
Unfortunately, the sliding-fit plunger assembly has proven to inherently limit the design, operation, and overall service life of conventional solenoid control valves. These limitations are primarily due to the free-floating movement of the plunger. For example, due to the frictional force induced between the outer wall of the plunger and the inner wall of the core spool during operation of the flow control valve, a greater magnetic force is required to drive the plunger through the bore hole. To facilitate this greater magnetic force, a larger solenoid coil and corresponding solenoid housing are typically required.
Another detrimental side effect of the sliding-fit plunger assembly is the production of wear products or particulates. These wear products are generated by the outer wall of the plunger abrasively sliding against the inner wall of the core spool. Inherently the wear products increase the relative friction between sliding surfaces of the plunger and core spool, thus further necessitating a larger solenoid coil assembly. Likewise, as the wear products progress through the flow control valve, they tend to interfere with the sealing interface of the valve so as to create leakage risks, and may eventually become lodged between moving surfaces of the valve, thus rendering the valve inoperative.
Additionally, the sliding-fit plunger's freedom of radial and rotational movement typically results in the degradation of the sealing interface. Due to the annular gap between the plunger and the bore hole, the sliding-fit plunger is not capable of consistent alignment with the sealing interface from one cycle to the next. As a result, inconsistent overlying seal impressions are created in the sealing interface, which leads to seal wear and subsequent leakage of the valve.
To reduce friction between the sliding-fit plunger and the bore hole of conventional solenoid flow control valves, lubricants are frequently required. However, due to the risk of contaminating the fluid flow which is intended to be controlled by the valve, the use of lubricant is often precluded. Of course without the desired lubricant, the operation and service life the solenoid valve are severely impaired.
Due to these limits in design, operation, and service life, it is well recognized that conventional solenoid flow control valves are inadequate for use in the pulse width modulation servocontrol field. Pulse width modulation requires the rapid high-speed continuous cycling of the flow control valve. Clearly, the inherent risks and limitations of the conventional solenoid valve, as discussed above, limit its use in the pulse width modulation field if sufficient lubricant coatings or plating are not provided on the plunger or the coil to reduce friction and galling.